This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to alkaline-sulfonate waterflooding involving the injection of an inorganic polyphosphate and an alkali metal carbonate.
In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding techniques. A practical problem encountered in such waterflooding techniques resides in the adsorption of the surfactants upon the rock surfaces of the reservoir. In this regard the surfactants (or other solutes which may be present in the injected flooding medium) move through the reservoir by means of a chromatographic adsorption/desorption process. Since the rate of desorption is less than the rate of adsorption, the surfactants are transported through the reservoir at a rate lower than that of the aqueous carrier fluid. Thus, the surfactant concentration at the flood front where the injected water contacts the reservoir oil decreases with the result that the desired low interfacial tension is not achieved. To counter this, it has been proposed, as taught for example in U.S. Pat. No. 3,469,630 to Hurd et al., to employ inorganic polyphosphate wetting agents which act as sacrificial agents to satisfy adsorption sites within the reservoir, thus decreasing adsorption of the surfactant. The inorganic polyphosphates, such as sodium tripolyphosphate or tetrasodium pyrophosphate, normally are introduced in a pretreatment slug injected prior to the surfactant slug and also in the surfactant slug itself. As taught by Hurd et al., such inorganic polyphosphates may be employed as the sole sacrificial agent or may be employed in combination with a soluble carbonate such as sodium carbonate.
Another surfactant waterflooding technique is taught in U.S. Pat. No. 3,847,823 to Clark et al. which discloses the use of an anionic waterflood additive characterized as an overbased sulfonate. The overbased sulfonate is arrived at by adding a base component, such as an alkali metal hydroxide, carbonate or bicarbonate, to a sulfonate mixture composed of a water-soluble sulfonate having an average equivalent weight of about 200 to about 400 and an oil-soluble sulfonate having an equivalent weight of about 400 to about 600. The base component is added in an amount over that required to neutralize the sulfonate such that the ratio of the weight of excess base component to the weight of the sulfonate is within the range of 0.03-2.0 and desirably within the range of 0.20-1.0. Overbasing may be arrived at in situ or prior to injection. In the former case, the sulfonate derived from a pale oil extract may be injected into the formation and this sulfonate slug then followed by an effective amount of an alkali metal hydroxide or alkali metal carbonate solution. The neutralized sulfonate mixture may be present in an amount within the range of 0.5-25 weight percent and may contain a chelating agent such as trisodium phosphate monohydrate to chelate calcium and/or magnesium ions.
Another waterflooding procedure, in which surfactants are formed in situ, involves alkaline waterflooding. In this process an aqueous solution of an alkali metal or ammonium hydroxide or carbonate is injected in order to neutralize organic acids in the reservoir oil to produce the corresponding alkali metal or ammonium salts. An improved alkaline waterflooding process is described in U.S. Pat. No. 3,927,716 to Ralph F. Burdyn, Harry L. Chang, and William R. Foster, and entitled "ALKALINE WATERFLOODING PROCESS". In this process an aqueous alkaline solution is employed in which the alkalinity and monovalent salt salinity of the solution are controlled within defined ranges in order to result in low oil-water interfacial tensions which enhance the microscopic displacement of oil from the interstices of the reservoir rock. A thickened water slug may be used for the purpose of mobility control in the alkaline waterflood. As recognized in the Burdyn et al. application, the results achieved by alkaline waterflooding and the roles played by the various recovery mechanisms involved depend to some extent upon the molecular weight distribution of the organic acids within the reservoir oil.
An additional factor which is relevant to the efficacy of an alkaline waterflood is the total acid content of the reservoir oil. This is commonly measured by the "acid number" which is defined as the milligrams of potassium hydroxide required to neutralize the acids in one gram of crude oil in a nonaqueous-type titration. Various procedures have been proposed for the alkaline waterflooding of those reservoirs in which the crude oil is considered to have an inadequate acid content as indicated by the acid number. Air, peroxides, or other oxidizing agents may be injected into the reservoir in order to oxidize the oil in situ to form additional organic acids. Additionally, a preoxidized oil bank may be injected or high molecular weight acids may be added to the injected oil bank. In any case, an aqueous alkaline solution is injected in order to form the sodium salts of the organic acids.
A further alkaline waterflooding technique which is useful in reservoirs containing oil exhibiting low to intermediate acid numbers is set forth in U.S. Pat. application Ser. No. 570,686, filed Apr. 23, 1975, by Ralph F. Burdyn, Harry L. Chang, and Evin L. Cook, entitled "OIL RECOVERY BY ALKALINE-SURFACTANT WATERFLOODING".
In this process, the interfacial tension between the reservoir oil and the injected water is reduced to a desired low value through the use of a system comprising a sulfonate surfactant and also an alkaline agent which functions to convert the organic acids within the reservoir to the corresponding surface-active monovalent metal soaps. In this procedure, there is injected an aqueous initiation slug containing an alkaline agent selected from the group consisting of the alkali metal and ammonium hydroxides. Subsequent to the injection of the initiation slug an aqueous surfactant slug is injected. This slug contains an alkaline agent and also a sulfonate surfactant.
Yet another beneficial alkaline waterflooding process is disclosed in the aforementioned parent application Ser. No. 553,697. As described in that application, an inorganic polyphosphate is employed in conjunction with alkaline waterflooding in order to increase the amount of alkaline agent available for the in situ formation of surfactants by complexing divalent metal ions within the reservoir and buffering the pH of the alkaline solution as well as by satisfying adsorption sites within the reservoir. In this procedure, an aqueous pretreatment solution containing an inorganic polyphosphate is injected into the reservoir, and this pretreatment solution is followed by the injection of an aqueous alkaline solution. The alkaline solution may also contain an inorganic polyphosphate which in most cases will be in a somewhat lower concentration than the polyphosphate concentration of the pretreatment solution.